The present invention relates to an active anti-vibration apparatus, such as for example, an air spring type active anti-vibration apparatus which employs air spring actuators and which is ideally used as a component unit of a semiconductor exposure apparatus with an exposure XY stage mounted thereon.
A group of equipment such as an optical microscope and an exposure XY stage which are susceptible to vibration is mounted on an anti-vibration table. The exposure XY stage, in particular, must be mounted on an anti-vibration table which is able to eliminate vibration transmitted from outside as much as possible in order to permit proper and quick exposure because exposure has to be carried out with the exposure XY stage completely stopped and still. The exposure XY stage has an operation mode in which the exposure XY stage performs intermittent movement known as "step-and-repeat". In this step-and-repeat mode, the stage itself repeatedly generates stepping vibration; therefore, considerations should be given to the vibration of the anti-vibration table caused by the foregoing stepping vibration.
If the vibration cannot be completely settled and if it remains, then exposure operation cannot be initiated. For this reason, the anti-vibration table is required to provide vibration controlling performance for eliminating external vibration and the vibration attributable to the motion of mounted equipment in a well balanced manner.
In recent years, a scan-type semiconductor exposure apparatus which irradiates exposure light to a silicon wafer while scanning an XY stage or the like is beginning to replace the step-and-repeat type semiconductor exposure apparatus which brings the XY stage to a complete halt, then irradiates exposure light to the silicon wafer mounted on the stage. The anti-vibration table used for such a scan-type apparatus is also required to provide controlling performance for eliminating external vibration and to control the vibration resulting from the motion of mounted equipment in a well-balanced manner.
As it is widely known, the anti-vibration tables are classified into passive type and active type. Recently, there has been a trend toward using active anti-vibration apparatuses to respond to the demand for highly accurate positioning, highly accurate scanning, quicker movement, etc., required of the equipment mounted on an anti-vibration table. Air springs, voice coil motors, piezo elements, etc., are known as the actuators employed for such anti-vibration apparatuses. A specific description will be given herein of an active anti-vibration apparatus as a conventional example using air springs as the actuators.
First, referring to FIG. 11, the configuration and operation of the active anti-vibration apparatus of the prior art, which employs air springs as the actuators, will be described. In FIG. 11, an anti-vibration table 1 on which precision equipment is mounted has active support legs 2a through 2d.
The active support legs 2a through 2d are primarily composed of air spring actuators which include vibration measurers 3a through 3d for measuring the vibration in the horizontal direction, air spring actuators 4a through 4d which include servo valves (not shown) for generating drive power in the horizontal direction, and displacement sensors 6a through 6d for measuring horizontal displacement. Acceleration sensors, geophone sensors, etc., may be used as the vibration measurers 3a through 3d. As the displacement sensors 6a through 6d, eddy current type displacement sensors, capacitive sensors, position sensors employing photoelectric converters, etc., may be used.
As shown in the drawing, the active support legs 2a through 2d which include, as primary composing elements thereof, the foregoing air spring actuators 4a through 4d, the displacement sensors 6a through 6d, and vibration measurers 3a through 3d, are disposed at the four corners of the anti-vibration table 1 to support the anti-vibration table 1 and the precision equipment mounted thereon. Although the main composing elements which perpendicularly support the anti-vibration table 1 are not shown, they share the same configuration as the aforesaid composing elements which horizontally support the anti-vibration table.
The configuration and operation of a decoupling feedback apparatus in each motional mode for the active support legs 2a through 2d of the control scheme of the active anti-vibration apparatus will now be described.
First, electrical output signals Aa through Ad of the vibration measurers 3a through 3d of an acceleration sensor or the like are supplied to a motional-mode selector 7A related to the acceleration for selecting the translation in the x-direction, the translation in the y-direction, and the rotation about the z-axis of the anti-vibration table 1. Based on the received electrical output signals, the motional-mode selector 7A issues motional mode acceleration signals (a.sub.x, a.sub.y, and a.theta.z) which are turned into negative feedback signals related to the acceleration for each motional mode via gain compensators 17.sub.x, 17.sub.y, and 17.sub.z which have appropriate amplification degrees and time constants; the negative feedback signals are sent back to the section preceding a motional-mode distributor 9. This acceleration feedback loop provides a damping operation for each motional mode to stabilize the operation of the anti-vibration table 1.
The electrical output signals Za through Zd of the displacement sensors 6a through 6d are supplied to error amplifiers 10a through 10d. Another type of inputs is supplied to the respective error amplifiers 10a through 10d from a position target voltage input terminal 11. The voltage applied to the input terminal represents the equilibrium position of the anti-vibration table 1 with respect to the foundation such as a floor on which the active support legs 2a through 2d are installed. Position error signals e.sub.a through e.sub.d which are the outputs of the error amplifiers 10a through 10d are applied to a motional-mode selector 7P so that the motional-mode selector 7P related to displacement issues motional mode error signals (s.sub.x, s.sub.y, and s.theta.z) which are then applied to PI compensators 12.sub.x, 12.sub.y, and 12.theta.z for setting the steady-state error to zero for each motional mode, thus making up a positional feedback loop. In this case, P denotes proportion and I denotes integrating operation.
Next, the output signals of the PI compensators 12.sub.x, 12.sub.y, and 12.theta.z and the negative feedback signals related to the acceleration by motional mode which are issued from the gain compensators 17.sub.x, 17.sub.y, and 17.sub.z are added to provide drive signals (d.sub.x, d.sub.y, and d.theta.z). The drive signals (d.sub.x, d.sub.y, and d.theta.z) by motional mode are applied to the motional-mode distributor 9 which generates drive signals to be sent to the respective axes. When the output signals (d.sub.a, d.sub.b, d.sub.c, and d.sub.d) of the motional-mode distributor 9 are supplied to voltage-current converters 8a through 8d of the respective axes, the servo valves (not shown) are opened or closed to regulate the internal pressures of the air spring actuators 4a through 4d; the changes in the internal pressures cause the applied voltage supplied from the position target voltage input terminal 11 to maintain the anti-vibration table 1 at a predetermined position without a steady-state error.
FIG. 12 is a top plan view showing the layout of the active support legs 2a through 2d related to the anti-vibration apparatus shown in FIG. 11. The twin-headed arrows entered in the respective support legs in the drawing indicate the directions in which the air spring actuators 4a through 4d can be driven and they also indicate the measurement directions of the vibration measurers 3a through 3d and the displacement sensors 6a through 6d. It can be easily understood that providing such driving directions and measuring directions permits the control of the translation in the x-direction, the translation in the y-direction, and the rotation about the z-axis.
Referring back to FIG. 11, an XY stage 13, which is a precision machine, is mounted on the anti-vibration table 1. The behavior of the XY stage 13 when it performs the step-and-repeat operation in the x-direction while repeatedly reversing the moving direction will be described, although the detailed description of the structure of the XY stage 13 will be omitted. The XY stage 13 mainly generates the translation in the x-direction and the rotary motion in the y-direction. For the rotary motion in the y-direction, the air spring actuator in the z-direction (not shown) generates driving power to eliminate and control the vibration of the stage. When the XY stage 13 reverses the direction of motion thereof, it is driven in the x-direction and y-direction, so that a drive reaction force is generated to produce a rotary motion about the z-axis. The observation result of the deformation or distortion of the structure including the anti-vibration table 1 has revealed that the deformation at that time is not negligible in ensuring the positioning accuracy of the XY stage 13 or wafer exposure for which a nanometer-level accuracy is required. As mentioned above, zeroing the acceleration and restoring and maintaining the anti-vibration table 1 to the equilibrium position are ensured by the acceleration and position feedback loops. The aforesaid deformation is caused by the failure to control the internal pressures of the air spring actuators 4a through 4d to a predetermined pressure level. This means that no consideration has been given to the changes in pressure. The changes in the internal pressures of the air spring actuators 4a through 4d are the changes in force, and the application of an imbalanced force results in the deformation of the structure including the anti-vibration table 1. The deformation disturbs the reading of measuring equipment (not shown) mounted on the structure including the anti-vibration table. For instance, if the apparatus shown in FIG. 11 is a semiconductor exposure apparatus, the deformation leads to distorted exposure of a wafer, resulting in markedly low productivity.
As a publicly known example of a solution to the problem described above, there is an anti-vibration apparatus disclosed in Japanese Patent application Laid-Open No. 8-166043. This anti-vibration apparatus, which is equipped with a plurality of load sensors, is characterized in that it controls the adjustment made by a height adjusting means in accordance with detection results of the load sensors so as to maintain the balance of a reaction force received from anti-vibration pads. The object of the apparatus is to solve the problem of the deteriorated positioning accuracy or the like of the stage on a surface plate, namely, an anti-vibration table, by controlling or reducing the deformation of the surface plate attributable to the reaction force. The technical means for restraining the deformation of a structure disclosed in the publication, however, is adapted to control the adjustment of vertical motion devices (3A through 3D) serving as the height adjusting means in accordance with the measurements of the load sensors (5A through 5D) by using signals described in the publication in order to maintain the balance of the reaction force received from vibration pads unchanged. This means that the measurements of the load sensors are employed as the target values supplied to a closed-loop system which is constituted by the height adjusting means serving as actuators. It is understood that the apparatus does not have a minor loop wherein the measurements of the load sensors are fed back in the closed-loop system. Further, electric and pneumatic dampers which are screw-driven are enumerated as the vertical motion devices. In the latter type, the target values to be supplied to the closed-loop system of a pneumatic damper are generated in accordance with the measurements of the load sensors and the generated target values are applied; however, the system responds very slowly. Hence, the apparatus has not been successful in restraining the changes in load resulting especially from the short-cycle intermittent motion of the step-and-repeat operation.
There has also been a prior art arrangement known as "stage reaction force feedforward art" in which signals synchronized with the driving timing of the XY stage 13 are subjected to appropriate signal processing to conduct feedforward on the active anti-vibration apparatus in order to restrain the reaction force produced when the stage is driven. There has been another prior art arrangement known as "floor vibration feedforward art" wherein the vibration of a floor is detected by using an appropriate floor vibration detecting means and the resulting signals are subjected to appropriate signal processing so as to carry out feedforward on the active anti-vibration apparatus thereby to control the transmission of the vibration of a foundation such as a floor on which the apparatus is installed onto the anti-vibration table via mechanism members. In both feedforward devices, however, it is required to design feedforward compensators by taking the drive characteristics of the active anti-vibration apparatus into account.
The conventional active anti-vibration apparatuses described above are provided with an acceleration feedback loop for imparting the damping effect and a positional feedback loop for maintaining the posture. However, the force applied to the structure including the anti-vibration table 1 supported by the active support legs 2a through 2d is not placed under the control by the feedback loops. Hence, if a mounted precision machine, e.g., the XY stage 13, performs sudden acceleration or deceleration on the anti-vibration table, a resulting intense reaction force is generated and causes a great change in the load on the anti-vibration table. This results in distortion of the structure. The absence of a feedback loop for compensating for the changes in pressure or load means the absence of a control means, thus adversely affecting the efforts in achieving zero acceleration and positional error signals in a steady state wherein the feedback loop exists. The deformation of the structure disturbs the reading on measuring equipment (not shown) mounted thereon or the reading on a laser interferometer (not shown) required for positioning the XY stage 13, leading to deteriorated exposure accuracy with resultant considerably poor productivity.
Another conventional example will be explained with reference to FIG. 15 which illustrates the configuration and operation of an active anti-vibration apparatus of the prior art, which employs air springs as the actuators. In FIG. 15, an anti-vibration table 101 on which precision equipment is mounted has active support legs 102a through 102c.
The active support legs 102a through 102c are primarily composed of air spring actuators which include vibration measurers 103a through 103c for measuring the vibration in the horizontal direction, air spring actuators 104a through 104c which include servo valves (not shown) for generating drive power in the horizontal direction, and displacement sensors 105a through 105c which measure horizontal displacement. Acceleration sensors, geophone sensors, etc., may be used as the vibration measurers 103a through 103c. As the displacement sensors 105a through 105c, eddy current type displacement sensors, capacitive sensors, position sensors employing photoelectric converters, etc., may be used.
As shown in the drawing, the active support legs 102a through 102c which include, as primary composing elements thereof, the foregoing air spring actuators 104a through 104c, the displacement sensors 105a through 105c, and vibration measurers 103a through 103c, are disposed at the three corners of the approximately triangular anti-vibration table 101 to support the anti-vibration table 101 and the precision equipment mounted thereon. Although the main composing elements which perpendicularly support the anti-vibration table 101 are not shown, they share the same configuration as the aforesaid composing elements which horizontally support the anti-vibration table.
The configuration and operation of a decoupling feedback apparatus in each motional mode for the active support legs 102a through 102c of the control scheme of the active anti-vibration apparatus will now be described.
First, electrical output signals Aa through Ac of the vibration measurers 3a through 3c of an acceleration sensor or the like are supplied to a motional-mode selector 106A related to the acceleration for selecting the translation in the x-direction, the translation in the y-direction, and the rotation about the z-axis of the anti-vibration table 1. Based on the received electrical output signals, the motional-mode selector 106A issues motional mode acceleration signals (a.sub.x, a.sub.y, and a.theta.z) which are turned into negative feedback signals related to the acceleration for each motional mode via gain compensators 107.sub.x, 107.sub.y, and 107.theta.z which have appropriate amplification degrees and time constants; the negative feedback signals are sent back to the section before a motional-mode distributor 111. This acceleration feedback loop provides a damping operation for each motional mode to stabilize the operation of the anti-vibration table 101.
The electrical output signals Za through Zc of the displacement sensors 105a through 105c are supplied to error amplifiers 108a through 108c. Another type of inputs is supplied to the respective error amplifiers 108a through 108c from a position target voltage input terminal 109. The voltage applied to the input terminal 109 represents the equilibrium position of the anti-vibration table 101 with respect to the foundation such as a floor on which the active support legs 102a through 102c are installed. Position error signals e.sub.a through e.sub.c which are the outputs of the error amplifiers 108a through 108c are applied to a motional-mode selector 106P related to displacement so that the motional-mode selector 106P issues motional mode error signals (S.sub.x, S.sub.y, and S.theta.z) which are then applied to PI compensators 110.sub.x, 110.sub.y, and 110.theta.z for setting the steady-state error to zero for each motional mode, thus making up a position feedback loop.
Next, the output signals of the PI compensators 110.sub.x, 110.sub.y, and 110.theta.z and the negative feedback signals related to the acceleration by motional mode which are issued from the gain compensators 107.sub.x, 107.sub.y, and 107.sub.z are added to provide drive signals (d.sub.x, d.sub.y, and d.theta.z) by motional mode. The drive signals (d.sub.x, d.sub.y, and d.theta.z) by motional mode are applied to the motional-mode distributor 111 which generates drive signals to be sent to the respective axes. When the output signals (d.sub.a, d.sub.b, and d.sub.c) of the motional-mode distributor 111 energize voltage-current converters 112a through 112c of the respective axes, the servo valves (not shown) are opened or closed to regulate the internal pressures of the air spring actuators 104a through 104c; the changes in the internal pressures cause the applied voltage supplied from the position target voltage input terminal 109 to maintain the anti-vibration table 101 at a predetermined position without a steady-state error. Reference numeral 140 denotes an XY stage, which is precision equipment, although a detailed description thereof will be omitted.
FIG. 16 shows an example of the mechanical structure of an active support leg 102. In FIG. 16, reference numeral 113 corresponds to the anti-vibration table 101 of FIG. 2, and reference numeral 114 denotes a fastening plate for firmly joining the active support leg 102 and the anti-vibration table 113. The active support leg 102 includes an air spring 115V (H) of the perpendicular or horizontal direction, a multi-layer rubber member 116, an acceleration sensor 117V (H) of the perpendicular or horizontal direction, a position sensor 118V (H) of the perpendicular or horizontal direction, a servo valve 119V (H) of the perpendicular or horizontal direction for allowing air, which is a working fluid, to go in and out of the air spring, a pre-load coil spring 120 of the horizontal direction, a force pin 121, and a frame 122. Reference numeral 123 indicates a floor with which the active support leg 102 is in contact. When the active support leg 102 having such a mechanical structure is used to support the anti-vibration table 101, the following problems are posed.
(1) The vibration of the floor on which the active support leg 102 is installed is transmitted to the anti-vibration table 101 or 113 via the composing members. It cannot be expected, therefore, to achieve further improvement of the transmissibility, which is the transfer function of the vibration on the anti-vibration table 101 with respect to the floor vibration.
(2) In the case of a semiconductor exposure apparatus, the XY stage, which is a precision machine, is mounted on the anti-vibration table. When the XY stage is actuated, the reaction force from the repetitive operation thereof continues to be applied to the anti-vibration table 101. At this time, the actuator device in the active support leg 102 generates a driving force against the reaction force, causing the driving force to be applied also to the composing members. This in turn causes the composing members, which have been installed with limited rigidity, to deform or it generates local mechanical vibration, adversely affecting the performance of the semiconductor exposure apparatus.
(3) The driving force of the air spring actuator generated according to the reaction force arising from driving the XY stage causes the structure including the anti-vibration table to deform, spoiling the performance of the exposure performance of the semiconductor exposure apparatus.
To solve the problems described in (1) and (2) above, the active support leg must have a structure which minimizes the need for using the composing members so that the route along which the vibration of the floor is directly transmitted is cut off with resultant improved transmissibility with respect to the vibration of the floor. The absence of composing members makes it possible to obviate the deformation and mechanical resonance of the composing members which are caused by the reaction force generated by sudden acceleration of the XY stage on the anti-vibration table.
As an example of the configuration of the active support leg without the composing members, there has been known a so-called "push-pull disposition" wherein actuators, which are oriented in the direction of the operating axis and driven in the directions opposite from each other, are opposed to each other. The actuators disposed in the push-pull layout are known in the art. There is another example of the push-pull disposition in addition to that of the active anti-vibration apparatus. For example, a magnetic bearing shown in FIG. 17 supports a movable member 125 without touching it by using a pair of an electromagnet 124R and electromagnet 124L which are horizontally disposed against each other. In general, steady current is supplied to the electromagnets 124R and 124L; if the movable member 125 is displaced to the right, then a controller (not shown) supplies more current to the left electromagnet 124L, whereas it supplies less current to the right electromagnet 124R according to the displacement. This enables the movable member 125 to be placed in a predetermined position without contact.
Referring back to the case of the active anti-vibration apparatus, there is an active damping table disclosed in Japanese Patent application Laid-Open No. 3-219141 as an example of the prior art of the apparatus configuration in which air spring actuators are disposed in the push-pull layout. According to this example of the prior art, in the block circuit diagram of a first embodiment, a pair of right and left air springs are disposed such that the air from a pneumatic source is applied to one air spring at a predetermined bias via a servo valve and that the internal pressure of the other air spring is changed in response to control signals. According to the block circuit diagram of a second embodiment of the prior art, a pair of right and left air springs are configured such that control signals of 180 degrees reversed in phase are supplied to the respective air springs to control the two air springs in the same direction so as to double the control power.
From the discussion above, it can be seen that an active support leg should have air spring actuators disposed in the push-pull layout so as to provide the actuators with pressure feedback or load feedback in order to further improve transmissibility, restrain or reduce the deformation and mechanical resonance attributable to the drive force generated by active support legs, and control the deformation of a structure including an anti-vibration table. It has not yet been known, however, about a configuration which permits such pressure feedback or load feedback to the active support leg having a pair of air spring actuators.
It has been proposed to employ an active support leg having the push-pull structure wherein the air spring actuators are disposed against each other so as to control the transmission of the vibration of a floor and also to obviate the occurrence of local vibration or deformation in the active support leg caused by the drive reaction force of the XY stage.
However, the minimum necessary feedback structure for controlling the deformation of a structure caused by sudden acceleration of an XY stage in a horizontal direction and the structure of a control scheme for conducting pressure or load feedback to an active support leg having the push-pull structure have not yet been clearly established and they have not been implemented.